Sinter-hardening powder and their sintered compacts

ABSTRACT

A sinter-hardening powder can yield a sintered compact with high strength, high hardness, and high density. A raw powder for sintering includes Fe as its primary component and also comprising 0.1-0.8 wt % C, 5.0-12.0 wt % Ni, 0.1-2.0 wt % Cr, and 0.1-2.0 wt % Mo, wherein the mean particle size of the raw powder for sintering is 20 μm or less. The sintered and tempered compact, without any quenching treatment, has high hardness, high strength, high density, and good ductility.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of a prior application Ser. No. 11/308,824, filed on May 11, 2006. The prior application is a continuation-in-part application of application Ser. No. 10/907,155, filed on Mar. 23, 2005, now abandoned, which claims the priority benefit of Taiwan application serial no. 93116634, filed on Jun. 10, 2004 and Taiwan application serial no. 93126297, filed on Sep. 1, 2004. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to sinter-hardening powders, in particular, to compositions useful for yielding high hardness, high strength, and high density in sintered parts by the metal injection molding process or the press-and-sinter process.

2. Description of Related Art

To attain high hardness and high strength, most sintered components require a heat treatment, such as quenching, in order to form martensite. However, when performing quenching, several problems such as deformation, size inconsistency, or cracking may occur due to the volume expansion when the part transforms from austenite to martensite, or due to the thermal stress caused by the fast cooling of the quenching treatment. In addition, performing heat treatment on the components incurs additional costs. Therefore, sinter-hardening powders have been developed in the press-and-sinter process by adding carbon and high hardenability alloying elements such as molybdenum, nickel, manganese, and chromium to iron powders, pressing out the green compact through the conventional compacting process, and then sintering the green compact into a finished product with a hardness above HRC30. Examples of alloys produced by this method are Ancorsteel 737SH (Fe-0.42Mn-1.40Ni-1.25Mo-0.5C) and Ancorsteel 4300 (Fe-1Cr-1Ni-0.8Mo-0.6Si-0.1Mn-0.5C) powders from Hoeganaes Corp., and ATOMET 4701 (Fe-0.45Mn-0.90Ni-1.00Mo-0.45Cr—C) from Quebec Metal Powders Limited. The components made from these powders are required to cool at a minimum of 30° C. per minute in the sintering furnace in order to generate martensite. Another sinter-hardening powder is disclosed in U.S. Pat. No. 5,682,588, which relates to a powder mixture created by adding 1 to 3 wt % of Ni, 1 to 2 wt % of Cu, and graphite powders to a prealloyed powder with a composition consisting of 3 to 5 wt % Ni, 0.4 to 0.7 wt % Mo, and the remainder Fe. The claimed powders are compacted, sintered between 1130-1230° C., and then cooled at a rate of 5-20° C./minute to attain the sinter-hardening effect. This may improve the process by lowering the minimum cooling rate of 30° C./min, as described in the previously mentioned processes. However, the strength and other properties, in particular the ductility and toughness, of these sinter-hardening powders are still unsatisfactory.

There are also several material standards for sinter-hardened alloys set forth by the Metal Powder Industries Federation (MPIF) for press-and-sinter compacts, examples being FLNC-4408 (1.0-3.0 wt % Ni, 0.65-0.95 wt % Mo, 1.0-3.0 wt % Cu, 0.05-0.3 wt % Mn, 0.6-0.9 wt % C, and the remaining portion is Fe) and FLC2-4808 (1.2-1.6 wt % Ni, 1.1-1.4 wt % Mo, 1.0-3.0 wt % Cu, 0.3-0.5 wt % Mn, 0.6-0.9 wt % C, and the remaining portion is Fe) prealloyed low-alloy steel powders. After sintering and tempering, the sinter-hardening FLC2-4808 steel can reach a tensile strength of 1070 MPa at a density of 7.2 g/cm³, and a hardness of HRC40, but the ductility is less than 1.0%. Another standard of sinter-hardenable prealloyed steel is the FL-5305 (Fe-3Cr-0.5Mo-0.5C), which can attain a tensile strength of 1100 MPa and a hardness of HRC 33 at a density of 7.3 g/cm³, but the ductility is less than 1.0%. Although these press-and-sinter alloys are of the sinter-hardening type, the mechanical properties are not satisfactory, and the required cooling rate is still very fast, at a minimum of 30° C./min. Thus, an additional high cooling rate system has to be installed in the sintering furnace. In addition, these high cooling rates, while slower than those of quenching in oil or water, are still fast enough to cause problems such as deformation, inconsistency of the dimensions, and quenching cracks. Thus, a new sinter-hardening alloy that can yield high hardness, high strength, high density, and good ductility and allows the use of a slow cooling rate is still very much desired.

The metal injection molding (MIM) process is a manufacturing method of producing sintered parts with high density and complicated shapes. In this process, mixed powders or prealloyed powders with a mean particle size less than 30 μm are kneaded with binders. The kneaded materials are injection molded into green compacts, which are then debinded and sintered. Since the diffusion distance is short, the added alloying elements can be homogenized easily in the matrix after sintering. Therefore, components sintered from the fine powders possess mechanical properties better than those of the traditional pressed-and-sintered components, which usually use powders with a mean particle size between 50 and 100 μm. At present, the alloys commonly used for metal injection molding are the Fe—Ni—Mo—C alloy series, exemplified by MIM-4605 (1.5-2.5 wt % Ni, 0.2-0.5 wt % Mo, 0.4-0.6 wt % C, <1.0 wt % Si, the remaining portion is Fe), which has the highest strength according to the MPIF standards. This alloy, after sintering, reaches a tensile strength of 415 MPa, a hardness of HRB62, and a ductility of 15%. To attain higher tensile strength and hardness, the sintered product has to be quenched and tempered. It then reaches a tensile strength of 1655 MPa, a hardness of HRC48, and a ductility of 2.0%.

Although mechanical properties of the metal injection molded products can be obtained by heat treatment after sintering, the costs of the heat treatment and the yield loss due to quenching increase the total production cost. Hence, it is desirable to improve the competitiveness of sintered compacts by using sinter-hardening powders. However, according to the Metal Powder Industries Federation Standards, no sinter-hardening alloys are listed for the metal injection molded products. Moreover, few patents have been disclosed to date on sinter-hardening powder that can attain strength, hardness, and density similar to those of the quenched-and-tempered compacts.

Although there are no sinter-hardened MIM standards, several steel powders, not sinter-hardening grade, have been disclosed for the metal injection molding process. In particular, U.S. Pat. No. 7,163,569 discloses an atomized prealloyed powder that can yield products with uniform mechanical properties and dimensions. The composition comprises 0.8 wt % or less of C, 0.05 to 1.0 wt % Si, 1.0 wt % or less of Mn, 0.15 to 1.0 wt % Nb, and 1.0 to 10.0 wt % Ni and further contains at least one element from among 2.0 wt % or less of Cr, 3.0 wt % or less of Mo, 3.0 wt % or less of Cu, and 0.05 wt % or less of Ti. This prealloyed powder, along with those specified in FLNC-4408 and FLC2-4808, is usually produced by gas or water atomization and is used when the distribution of the alloying elements is critical. One widely used prealloyed powder is stainless steel, in which the distribution of the Ni and Cr must be uniform so that the good corrosion resistance can be attained on all the surfaces of the part. Another example is tool steel, such as D2 steel, in which the distribution of the Cr, Mo, V, Co, and C must be uniform so that carbides can be uniformly dispersed in the matrix to provide good wear resistance. However, the production cost of fine atomized powders is expensive because only a small portion of the as-atomized powders can be used. Another disadvantage is the lack of flexibility in making alloys with special compositions. To produce parts with such customer-designed compositions, special orders for the prealloyed powder must be placed to a powder supplier, which usually means a long waiting time that delays the delivery of the final sintered products. In contrast, when mixed powders are used, alloys with special compositions can be produced by mixing the elemental powders, such as Ni, Mo, Cr, with the base iron powder. Since the amounts of the alloying elements added are small, the cost of stocking these alloying powders is low.

Furthermore, although prealloyed powder provides homogeneous alloying, such an effect causes poor compressibility and serious wear on the tooling when the compaction method is used in making the conventional press-and-sinter products. In the MIM process, the wear on the kneaders, molding machines, and molds are also more severe due to the higher hardness of the prealloyed powder.

Yet another difference is present in the alloy design between the prealloyed powder and mixed powder; some elements needed in the prealloyed powder may not be required in mixed powders. For example, when the atomization process is employed, some Mn and Si are usually added during melting to reduce the oxygen content and to facilitate the melt flow during atomization. This is also one of the reasons why most alloys that involve melting in the process contain Mn and/or Si. The addition of Mn and Si is, however, undesirable in powder mixtures because both Mn and Si are prone to oxidation during sintering unless the dew point is carefully controlled to a very low level. The quantity of the alloying elements is also different because slightly higher amounts of alloying elements are required for powder mixtures. The reason is that complete homogenization cannot be attained during sintering in practice. These examples demonstrate that there are different guidelines in designing the alloy compositions of mixed powders and prealloyed powders.

Due to these disadvantages of using prealloyed powders, only stainless steels, tool steels, and some other special alloys that require completely homogeneous alloying are usually produced with prealloyed powders. Most MIM alloy steels are produced with mixed elemental powders, which comprise iron powder as the base powder and elemental powder or prealloyed powder as the alloying powder. Moreover, most press-and-sinter parts, which require high green density, are produced with high compressibility powder, such as elemental powders or soft ferro-alloy powder, such as Fe—Mo, Fe—Cr, or Fe—Cr—Mo powder, which contains little carbon.

There are some known powder metal and MIM alloys with compositions similar to that of this invention. To attain improved tensile strength without undue loss of ductility, Marshall et al. disclosed in GB Patent 1009425 a metal powder mixture from which sintered steel articles may be made with a composition comprised of 1-4.9 wt % Ni, 0.1-2 wt % Mn, 0.1-5 wt % Mo, 0.1-1 wt % C, and the remaining being iron plus the usual impurities. It further discloses that up to 5 wt % of the iron can be replaced by one or more other elements which do not adversely affect the tensile strength and ductility of the sintered parts. The list of the elements and the upper limits include 1 wt % Al, 0.3 wt % B, 5 wt % Cr, 5 wt % Cu, 1 wt % Mg, 4 wt % Nb and/or Ta, 0.3 wt % P, 1 wt % Si, 2 wt % Ti, 4 wt % W, 0.3 wt % V, 0.6 wt % Zr, 0.6 wt % Se, and 0.5 wt % Pb. To attain the full advantage of this composition, GB Patent 1009425 also states that mixed powders should be used, instead of prealloyed powders, because considerably better mechanical properties can be obtained. The base powder is coarse iron powder with a mean particle size of about 50 μm. For producing sintered components, the mixed powders are pressed, debinded, and sintered between 1200° C.-1400° C. However, the best tensile strength is only 1200 MPa for the Fe-4.9Ni-0.5Mn-1Mo-0.6C alloy.

The other alloy similar to the powder disclosed in this invention is described in U.S. Pat. No. 7,163,569, which presents a prealloyed powder for sintering that can yield a sintered MIM part with precise dimensions and uniform properties. The mean particle size is 8 microns or less. However, the prealloyed powder has many disadvantages as described above, such as low sintered density, low strength, and low hardness. Table 1 shows the densities of Fe-2Ni-0.3C and Fe-8Ni-0.8Cr-0.8Mo-0.5C compacts sintered at 1190 and 1320° C. using mixed powder and prealloyed powder. The hardness is measured in the as-sintered compact without tempering. In the first group of specimens, the mixed powder with carbonyl iron powder as the base powder is used. The Ni and Mo are added using elemental powders while the Cr is added in the form of Fe—Cr—Ni prealloyed powder. No graphite is added because carbonyl iron powder contains carbon. The second group of specimens used prealloyed powders. The results show that higher sintered densities are obtained by using mixed powders when sintered at both 1190 and 1320° C. In particular, the carbonyl iron powder mixture is sintered to 7.52 g/cm³ at a low temperature of 1190° C. This example shows the poor sinterability of prealloyed powders. To attain high sintered densities, prealloyed powder must be sintered at high temperatures, which is expensive due to the high energy cost and the high capital investment of high temperature sintering furnace. This has become a major disadvantage, particularly where energy consumption is a great concern. Furthermore, high temperature sintering causes coarser grains, which impair the mechanical properties. Table 1 also shows that higher hardness is obtained with mixed powder than with prealloyed powder. The information on the microstructure of the compact with mixed powder also showed that quite uniform microstructure is obtained because fine carbonyl iron powder is used and the diffusion distance is thus very short.

The above background emphasizes that a MIM compact sintered at low sintering temperatures and having a high hardness and high density requires a combination of carefully selected base powder, powder size, type of the alloying powder, i.e., elemental powder or ferro-alloy powder, and a well designed combination of high hardenability alloying elements. Such a combination is not easy to design or select even by people who are familiar with the skill and practice of the press-and-sinter and metal injection molding process. This is why there have been few sinter-hardening alloys that can obtain hardness and strength comparable to those obtained in this invention. The properties obtained in this invention do not require any post-sintering quenching treatment. Only tempering is required after sintering.

Table 1 shows comparison of Fe—Ni alloys with prealloyed powder and mixed powder using carbonyl iron powder as the base powder.

TABLE 1 Composition Temperature Properties Mixed Prealloyed Fe—8Ni—0.8Mo—0.8Cr—0.5C 1190° C. Density 7.52 g/cm³ 7.20 g/cm³ Hardness 52HRC 28HRC 1320° C. Density 7.59 g/cm³ 7.35 g/cm³ Hardness 54HRC 49HRC Fe—2Ni—0.3C 1190° C. Density 7.50 g/cm³ 7.22 g/cm³ Hardness 69HRB 63HRB 1320° C. Density 7.56 g/cm³ 7.32 g/cm³ Hardness 79HRB 74HRB

As mentioned above, application of fine powders improves the uniformity of the alloying elements and mechanical properties of the products. However, application of fine powders in the traditional press-and-sinter process is difficult because of the poor flowability of the powder, which in turn makes it difficult to fill the powders into the die cavity, and thus automated pressing cannot be used. However, this problem can be overcome by granulating the fine powders into large spherical particles, such as by the spray drying process, and the granulated powders can then be applied in the press-and-sinter process.

In view of the background described above, the present invention provides a raw powder mixture or granulated powder, by which sintered compacts with high hardness, high strength, and high sintered density can be attained immediately after sintering without the need for quenching.

SUMMARY OF THE INVENTION

This invention has solved the above-mentioned problems as a result of the carefully selected combination of the base powder and the optimum amounts and types of the alloying elements. The powder mixture or granulated powder uses elemental iron powder, such as atomized, reduced, or carbonyl iron powder, as the base powder. The mean particle size is 20 μm or less. The alloying elements includes 0.1-0.8 wt % of C, 5.0-12.0 wt % Ni, 0.1-2.0 wt % Cr, 0.1-2.0 wt % of Mo. The above composition can further contain at least one other minor strengthening element in the amount of 5.0 wt % or less. The strengthening elements can be selected from the group consisting of Cu, Mn, Si, Ti, Al, and P. The carbon can be provided by adding graphite or carbon black, or using carbon-containing carbonyl iron powders. The mixed powder is intended for the production of MIM compacts. The granulated powder is intended for the production of press-and-sinter compacts. Furthermore, the present invention provides a sintered compact by using the powder mixture or the granulated powder. The compact can be sinter-hardened at a normal furnace cooling rate of 3-30° C./minute in the sintering furnace without the need for fast cooling, which is required for other sinter-hardening powders. The sintered compact does not require any quenching treatment. Only low temperature tempering is needed to obtain optimum mechanical properties. The sintered body with the above-mentioned sinter-hardening powder has unprecedented hardness, tensile strength, and good ductility. Since no quenching process is required, the production cost is lower. A higher production yield is also attained due to the elimination of defects, such as cracks and distortions, which occur during quenching.

The present invention is achieved in view of the conventional problems such as those described above for press-and-sinter and metal injection molded products. A raw sinter-hardening powders or granulated sinter-hardening powders is provided for sintering whereby the sintered compacts can simultaneously achieve high strength, high hardness, and high density with a slow cooling rate after sintering and without any quenching treatment.

In particular, this invention relates to a powder mixture and granulated powder that uses fine iron powder as the base powder, such as atomized, reduced, or carbonyl iron powder, with a mean particle size of 20 μm or less. The alloying elements include 0.1 to 0.8 wt % C, 5.0 to 12.0 wt % Ni, 0.1 to 2.0 wt % Cr, and 0.1 to 2.0 wt % of Mo. The powder may further contain at least one element from among 2 wt % or less Cu, 1 wt % or less Ti, 1 wt % or less Al, 1 wt % or less Mn, 1 wt % or less Si, and 1 wt % or less of P, wherein up to 5 wt % of the iron content can be replaced by one or more of these elements. The compact produced with this powder can be sintered and subsequently furnace-cooled at a slow cooling rate of 3° C./min and still attain a hardness greater than HRC30.

It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and they are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 shows that when the Cr content increases, the hardness of the sintered compact produced by using the metal injection molding process and using fine carbonyl iron powders increases first, reaching a maximum at about 0.7 wt %, and then decreases.

FIG. 2 shows that when the Cr content increases, the hardness of the sintered compact produced by using press-and-sinter process and using coarse iron powders increases first, reaching a maximum at about 3 wt %, and then decreases.

DESCRIPTION OF THE EMBODIMENTS

To produce economically sinter-hardened compacts with high strength, high hardness, and high density, several criteria for the raw powder must be met simultaneously. These criteria include: (a) containing elements that have high hardenability, (b) comprising fine particles so that high density and good homogeneity of the alloying elements can be attained after sintering, and (c) containing elements in the optimum amounts and ratio so that high sinterability and thus high density can be attained using low sintering temperatures. To meet these requirements simultaneously, the characteristics and alloying compositions of the sinter-hardening powder must be carefully designed.

Carbon, manganese, silicon, chromium, molybdenum, and nickel are the most widely used elements used in the wrought alloy steels. However, not all these elements are suitable for MIM and press-and-sinter parts unless some preventive measures are taken. For instance, chromium is quite reactive and is prone to react with the residual oxygen or water vapor in the sintering atmosphere and form chromium oxides. Manganese is an even more reactive element and will react easily with the sintering atmosphere to form manganese oxides. Moreover, manganese and chromium have a very high vapor pressure and thus, when green compacts are sintered in vacuum, the amounts of manganese and chromium retained in the sintered parts are usually significantly reduced unless argon is backfilled to a certain level. Silicon is another reactive element and can form oxide more easily than Cr and Mn during sintering. In spite of these problems, these alloying elements could still provide good hardening effect if these elements are added in the form of ferro-alloy powder and use a well-controlled sintering atmosphere, such as with a low dew point, so that oxide formation can be prevented. One of the methods used in this invention is to add ferro-alloy powders, such as Fe—Cr, Fe—Mn, Fe—Si, and their derivatives, such as Fe—Cr—Ni, Fe—Cr—Ni—Mo, etc. in the base iron powder. The active Cr, Si, and Mn are prealloyed in the powder so that their activities are reduced as compared to using pure elements. As a result, these active elements can be dissolved in the iron matrix in a regular sintering furnace without forming oxides and provide the hardenability needed. In addition, the sintering atmosphere should have a high reduction power, such as with high hydrogen content and low water vapor content. When a vacuum furnace is used, the degree of vacuum must be high enough to prevent the formation of the oxides. Meanwhile, some backfilled inert gases should be added to reduce the loss of the elements which have high vapor pressures, such as Cr and Mn.

The sinter-hardening powder disclosed in this invention comprises 0.1-0.8 wt % of C, 5.0-12.0 wt % Ni, 0.1-2.0 wt % Cr, 0.1-2.0 wt % Mo, wherein the mean particle size of the raw powder for sintering is 20 μm or less. The powder may further contain at least one element from among 2 wt % or less Cu, 1 wt % or less Ti, 1 wt % or less Al, 1 wt % or less Mn, 1 wt % or less Si, and 1 wt % or less of P, wherein up to 5 wt % of the iron content can be replaced by one or more of these elements. The compact produced using this powder can be sinter-hardened to a hardness of HRC30 or higher, even with a slow cooling rate of 3° C./min.

Ni is an element that could yield high hardenability and could also give high toughness and elongation for sintered compacts. Moreover, Ni is a very effective additive in improving the sintered density and toughness of steel compacts compared to most other alloying elements, such as Cu, Mo, Cr, Mn, and Si. Thus, it is advantageous to add Ni in the sinter-hardening powder. In the present invention, the Ni content preferred is between 5 wt % and 12.0 wt %; since the sinter-hardening characteristics are not obvious when the Ni content is lower than 5 wt % or when the Ni content is higher than 12 wt %, the sinter-hardening effect diminishes because too much austenite will be retained after sintering. In another aspect, when the Ni content is greater than 12 wt %, the benefits obtained are limited, and the cost of the sintered parts increases.

It has been found that the distribution of Ni in sintered steels is usually not uniform due to its slow diffusion rate in iron. Another reason is that carbon has a fast diffusion rate into iron and can thus quickly penetrate into the core of the iron powder. When Ni diffuses toward the iron powder core, the carbon tends to repel Ni, since Ni increases the chemical potential of carbon. Thus, it is difficult for Ni to be homogenized in the Fe matrix, and as a result, its sinter-hardening benefits are lost. The Ni-rich areas thus formed are low in strength and hardness and become vulnerable sites under stresses. However, this invention has found that when Cr is present, the repelling effect between Ni and C is alleviated and the distribution of Ni becomes more uniform as has been demonstrated using X-ray mapping. With a more uniform Ni distribution and the elimination of the soft Ni-rich areas, the overall hardness and strength of the sintered compacts increase.

The chromium content used in this invention is between 0.1 and 2 wt %. With less than 0.1 wt % Cr, the hardenability effect is insignificant. When the Cr content is over 2 wt %, the amount of martensite will be reduced. The Cr may be added in the form of ferro-chromium powder or other complex ferro-chromium powders that contain other alloying elements. As described above, this will reduce the activity of Cr and ensure its effectiveness in improving alloying of the Ni into the iron matrix during sintering without forming chromium oxides.

Table 2 and FIG. 1 show that, using a mixture of fine carbonyl iron powder and other alloying powders according to the composition of Fe-8Ni-0.8Mo-xCr-0.5C (x varies from 0 to 3 wt %) and using the metal injection molding process, the hardness of the sinter-hardened and tempered specimens increases first, reaching a maximum at about 0.7 wt %, and then decreases as the amount of Cr increases. This means that the amount of 3 wt % Cr is too large for the homogenization of Ni, since the hardness is below HRC30. However, when coarse iron powders are used, the homogenization becomes more difficult. Thus, the amount of Cr that is required to homogenize Ni increases. This effect is demonstrated by using a mixture of coarse water atomized iron powders (with a mean particle size of 75 μm) and other alloying powders according to the composition of Fe-4Ni-0.5Mo-0.5C and using the press-and-sinter process.

Table 2 and FIG. 2 show that the hardness increases, reaches the maximum at about 3 wt % Cr, and then decreases as the amount of Cr increases. These examples show that the optimum Cr content needed for the homogenization of Ni and the hardness increases as the particle size of the iron powder increases. The reason is that with the coarser base iron powder, a longer time or a higher temperature is required for obtaining good homogeneity of the alloying elements. In addition to the particle size effect, the optimum Cr content further depends on the amount of Ni in the sintered compact. These examples demonstrate that to attain high hardness, a narrow composition range of the alloying elements is required, such as for the selected iron particle size and the amount of Ni chosen in this invention.

Table 2 shows effects of Fe particle size and Ni content on the optimum Cr content of Fe-8Ni-0.8Mo-0.5C and Fe-4Ni-0.5Mo-0.5C.

TABLE 2 Cr content, Steel Type wt % Hardness, HRC Fe—8Ni—0.8Mo—0.5C with 5 μm 0 42 carbonyl iron powder as the 0.2 45 base powder 0.5 48 0.7 48 0.8 48 1.2 46 1.5 43 2.0 37 3.0 29 Fe—4Ni—0.5Mo—0.5C with 75 μm 0 <20 (HRB89) atomized iron powder 0.5 21 1.5 30 3 39 4 36 5 32 6 26

In FIG. 1, with 5 μm carbonyl iron powder as the base powder, the hardness of the Fe-8Ni-0.8Mo-0.5C compact increases as the amount of Cr increases, reaching the maximum at 0.7 wt %. In FIG. 2, with 75 μm atomized iron powder as the base powder, the hardness of the Fe-4Ni-0.5Mo-0.5C compact increases as the amount of Cr increases, reaching the maximum at 3.0 wt %.

When cast or wrought bulk alloys or prealloyed powders are designed, the optimum contents of the alloying elements will be lower than in mixed powders since homogenization of the various alloying elements have been achieved during melting. Another reason of the lower alloying content of the cast and wrought alloys is that the raw material must be soft enough so that plastic deformation and other secondary operations, such as machining, can be performed to meet the dimensional specifications of the part. The optimum mechanical properties are then attained with quenching and tempering treatment. In contrast, the press-and-sinter and MIM process are net shaping processes. Thus, it will be an advantage if the part, which has attained the dimensions after sintering, can also be sinter-hardened without further quenching treatment. This eliminates the defects that frequently develop during quenching. The elimination of the quenching process also makes the sinter-hardening materials more economical and more competitive. Thus, the optimum composition designed in this invention for the sinter-hardening powder for press-and-sinter and MIM products is carefully designed and is different from the AISI and MPIF standard alloy steels and those patented prealloyed powders. The narrow composition range disclosed in this invention is effective in producing new and unexpected results.

Molybdenum is another effective alloying element, and the amount used in this invention is between 0.1 and 2 wt %. When the Mo content is over 2 wt %, the hardness will be reduced due to the insufficient amount of martensite produced in the sintered parts. Moreover, molybdenum is also an expensive element. With too much Mo addition, the cost of the sintered products will become too high and thus make it less competitive compared to those counterparts made by other manufacturing processes. The molybdenum can be added in the form of pure Mo powder, ferro-molybdenum powder, or a complex ferro-molybdenum powder that contains other alloying elements.

Manganese has a very high hardenability and only a small amount is needed. The amount of Mn used in the present invention is less than 1 wt %. When the amount of Mn is over 1 wt %, the material becomes too brittle. To reduce its activity and prevent oxide formation during sintering, manganese is added in the form of ferro-manganese powder, which may further contain other alloying elements.

Silicon also has a high hardenability, and the amount used in this invention is less than 1 wt %. When Si content is over 1 wt %, the sintered compact becomes too brittle and is not suitable for structural parts. To reduce the activity of Si and prevent oxide formation, the silicon may be added in the form of ferro-silicon powder, which may further contain other alloying elements.

The most economic and effective hardening element is carbon. Carbon can be supplied from graphite powders, such as in most press-and-sinter powder metallurgy parts and in some MIM parts. It can also be supplied from carbon black powders. The addition of graphite or carbon black adds an additional processing step and thus adds cost. Therefore, the carbon-containing fine carbonyl iron powder, not the carbon-free carbonyl iron powder, is preferred as the based powder in this invention. Since carbon is contained in the carbonyl iron powder, it is homogeneously distributed in the sintered part. Moreover, the carbon in the carbonyl iron powder is not present in the form of hard cementite and thus will not cause adverse effects on the compressibility of the powder.

The above alloying elements may be added in the elemental powder form, or in the ferro-alloy powder form. Alternatively, two or more of the alloying elements may be added to the base iron powder in the form of prealloyed or master alloy powder.

To simultaneously achieve high strength, high hardness, and high sintered density, the selection of the base powder is critical for several reasons. For example, fine powders have large surface area, which is the driving force for sintering, and thus high sintered density can be obtained. In addition, the homogenization of the alloying elements is also faster because of the shorter diffusion distance of fine powders. Thus, fine powder is preferred in this invention. The type of the base powder, i.e., elemental powder or prealloyed powder is also important. Based on the considerations and the disadvantages of the prealloyed powder as described above, low cost iron powders, such as atomized, reduced, and carbonyl iron powder, are selected as the base powder in this invention. When doped with carefully selected alloying elements and carefully designed amounts, much improved mechanical properties and sintered densities as compared to using prealloyed powders can be attained.

Example 1

To the Fe-0.8Mo-0.8Cr mixed powder, different amounts of elemental Ni powder are added. The base iron powder selected is carbonyl iron powder. The Mo powder is added in the elemental powder form. The Cr is added in the form of Fe-16 wt % Cr ferroalloy powder. The chemical compositions are shown in Table 3. The admixed powder is doped with 7 wt % of the binder, kneaded in a high shear rate mixer at 150° C. for 1 hour, and then cooled to room temperature to obtain the granulated feedstock. Thereafter, the granulated feedstock is filled into the injection molding machine to produce the tensile test bar (e.g. the standard tensile bar from the MPIF-50 standard). The tensile bar is debound under the procedure applied from the known arts in the industry to remove the binder, subsequently heated in the vacuum furnace at 1200° C. for two hours, and then furnace-cooled at a cooling rate of about 6° C./minute between 600° C. and 300° C. After sintering, the specimen is tempered at 200° C. for 2 hours. The carbon content obtained is about 0.48 wt % and the sintered density is about 95.7%. The hardness increases as the Ni content increases, reaching a maximum at about 8 wt % Ni.

Table 3 shows the effect of Ni content on the hardness of sinter-hardened Fe-0.8Mo-0.8Cr-0.48C compact.

TABLE 3 Sample No. Ni (wt %) Hardness (HRC) 1 0 14 2 1 18 3 2 20 4 4 32 5 5 39 6 6 44 7 8 48 8 10 45 9 12 37

Example 2

To the Fe-8Ni-0.8Cr mixed powders, different amounts of elemental Mo powder are added. The base iron powder selected is carbonyl iron powder. The Cr is added in the form of Fe-16 wt % Cr ferroalloy powder, while the Ni is added in the elemental powder form. The chemical compositions are shown in Table 4. The mixed powder is processed following those procedures described in EXAMPLE 1. The carbon content thus obtained is about 0.45 wt %, and the sintered density is about 95.7%. As shown in Table 4, the hardness increases as the Mo content increases, reaching a maximum at about 0.8 wt % Mo. When the Mo content is 6 wt %, the hardness decreases to HRC35.

Table 4 shows the effect of Mo content on the hardness of sinter-hardened Fe-8Ni-0.8Cr-0.45C compact.

TABLE 4 Sample No. Mo (wt %) Hardness (HRC) 10 0 40 11 0.1 42 12 0.5 46 13 0.8 48 14 2 43 15 4 39 16 6 35

Example 3

To the Fe-8Ni-0.8Mo mixed powder, different amounts of Cr are added in the form of Fe-16 wt % Cr ferroalloy powder. The base iron powder selected is carbonyl iron powder. Both Ni and Mo are added in the elemental powder form. The chemical compositions are shown in Table 5. The mixed powder is processed the same way as that described in EXAMPLE 1. The carbon content obtained is about 0.43 wt % and the sintered density is about 95.6%. As shown in Table 5, the hardness increases as the Cr content increases, reaching a maximum at about 0.8 wt % Cr. When the Cr content is 6 wt %, the hardness is lower than HRC35.

Table 5 shows the effect of Cr content on the hardness of sinter-hardened Fe-8Ni-0.8Mo-0.43C compact.

TABLE 5 Sample No. Cr (wt %) Hardness (HRC) 17 0 42 18 0.1 43 19 0.2 45 20 0.5 48 21 0.8 48 22 1.5 46 23 2.0 42 24 3.0 37 25 6.0 30

The benefits of adding Cr is shown above in Table 5. However, such benefits can be seen only when there is a good combination of alloying elements. For example, Fe-8Ni-0.5C has a sintered hardness of HRC 42. When 0.5 wt % Cr is added, the hardness decreases to HRC40. A further check on the strength also shows a decrease from 1750 MPa to 1500 MPa. This indicates that the Cr does not increase the hardness of Fe-8Ni-0.5C unless Mo is present.

Example 4

To the Fe-8Ni-0.8Mo-0.8Cr mixed powder, different amounts of Mn are added in the form of Fe—Mn ferroalloy powder. The Cr is added in the form of Fe-16 wt % Cr ferroalloy powder. The base iron powder selected is carbonyl iron powder. Both Ni and Mo are added in the elemental powder form. The chemical compositions are shown in Table 6. The mixed powder is processed the same way as that described in EXAMPLE 1 The carbon content obtained is about 0.52 wt %, and the sintered density is about 95.5%. As shown in Table 6, the hardness decreases slightly as the Mn content increases. This indicates that the addition of Mn, which is usually included in prealloyed powders and cast parts, is not essential for the sinter-hardening powder disclosed in this invention. However, as the Mn content increases, the tensile strength is similar to that of Mn-free compacts. This indicates that the addition of Mn may not cause adverse effects, depending on the application and required properties.

Table 6 shows the effect of Mn content on the hardness and tensile strength of sinter-hardened Fe-8Ni-0.8Mo-0.8Cr-0.52C compact.

TABLE 6 Sample No. Mn (wt %) Hardness (HRC) Tensile Strength (MPa) 26 0 48 1970 27 0.2 46 2000 28 0.3 46 1980 29 0.4 46 1970 30 0.6 45 1980 31 1.0 45 1980

Example 5

To the Fe-8Ni-0.8Mo-0.8Cr mixed powder, different amounts of Si are added in the form of Fe-20 wt % Si ferroalloy powder. The Cr is added in the form of Fe-16 wt % Cr ferroalloy powder. The base iron powder selected is carbonyl iron powder. Both Ni and Mo are added in the elemental powder form. The chemical compositions are shown in Table 7. The mixed powder is processed the same way as that described in EXAMPLE 1 The carbon content obtained is about 0.50 wt %. As the silicon content increases to 1 wt %, the sintered density decreases slightly from 95.7% to 95.0%. As a result, the hardness decreases slightly too. This indicates that the addition of Si, which is usually included in prealloyed powders and cast parts, is not essential for the sinter-hardening powder disclosed in this invention. However, with a silicon content as high as 1 wt %, the sintered compact can still attain a hardness greater of HRC45.

Table 7 shows the effect of Si content on the hardness of sinter-hardened Fe-8Ni-0.8Mo-0.8Cr-0.50C compact.

TABLE 7 Sample No. Si (wt %) Hardness (HRC) 32 0 48 33 0.2 48 34 0.3 47 35 0.4 46 36 0.6 46 37 1.0 45

Example 6

To the Fe-8Ni-0.8Mo-0.8Cr mixed powder, different amounts of Cu are added in the form of elemental powder. The Cr is added in the Fe—Cr ferroalloy powder form. The base iron powder selected is carbonyl iron powder. Both Ni and Mo are added in the elemental powder form. The chemical compositions are shown in Table 8. The mixed powder is processed the same way as that described in EXAMPLE 1. The carbon content obtained is about 0.51 wt % and the sintered density is about 95.6%. As shown in Table 8, the hardness decreases slightly as the Cu content increases. However, the tensile strength increases slightly as the Cu content increases to about 0.3 wt %. It then starts to decrease to a level similar to that of the Cu-free tensile bars when the Cu content reaches 1.5 wt %. This indicates that the addition of Cu could have positive effects, depending on the properties required.

Table 8 shows the effect of Cu content on the hardness and tensile strength of sinter-hardened Fe-8Ni-0.8Mo-0.8Cr-0.51C. compact.

TABLE 8 Sample No. Cu (wt %) Hardness (HRC) Tensile Strength (MPa) 38 0 48 1970 39 0.3 46 2060 40 0.5 46 2010 41 1.0 45 2010 42 1.5 45 1970

Example 7

The Fe-8Ni-0.8Mo-0.8Cr powder is processed the same way as that described in EXAMPLE 1, except that the sintering temperature is 1320° C. The carbon content obtained is about 0.42 wt %, and the sintered density is about 96.6%. The hardness after tempering is HRC 47, similar to the HRC48 of the compact sintered at 1190° C. The tensile strength measured is 2010 MPa, slightly higher than the 1970 MPa, which is obtained by sintering at 1190° C. The elongations of the tensile bars sintered at 1190 and 1320° C. are 3.4 and 4.1 wt %, respectively. These tensile strengths are significantly better than the data reported to date on metal injection molded or pressed-and-sintered parts. The elongation is also quite adequate for applications in structural parts. The examples given in Tables 1 to 8 illustrate the sinter-hardening effect of the raw powder of the present invention.

Example 8

The Fe-8Ni-0.8Mo-0.8Cr powder is processed the same way as that described in EXAMPLE 1, except that graphite and carbon black powders are used as the source of carbon. The carbon-free carbonyl iron powder is used as the base powder. The carbon content, density, and hardness yielded are similar to those attained using carbonyl iron powder as the source of carbon.

Example 9

The Fe-8Ni-0.8Mo-0.8Cr-0.45C powder is processed the same way as that described in EXAMPLE 1 except that the cooling rate is changed. Table 9 shows that the sintered compact with a cooling rate of 3, 6, and 30° C./min has similar hardness of about HRC 48 after tempering. When the compact is quenched to water and tempered, the hardness is 51. This demonstrates that the composition of this invention yields a very high hardenability and the compact can be sinter-hardened even with a cooling rate as slow as 3° C./min.

Table 9 shows the effect of cooling rate on the hardness and tensile strength of sinter-hardened Fe-8Ni-0.8Mo-0.8Cr-0.45C compact.

TABLE 9 Cooling rate Hardness Sample No. ° C./min HRC 43 3 47 44 6 48 45 30 48 46 Quenching 51

Example 10

The compositions of the sinter-hardening powder disclosed in this invention are results of optimum combinations of the alloying elements. Table 10 lists the hardnesses of some other MIM sintered-and-tempered compacts (Sample No. 47 to 57) using the sinter-hardening powder disclosed in this invention. A press-and-sinter compact with the composition of Fe-8Ni-0.8Cr-0.8Mo-0.4C is also prepared. The carbonyl iron powder is mixed together with elemental Mo and Ni powders and Fe—Cr ferroalloy powder. The mixed powders, water, and binders (e.g.: Polyvinyl alcohol) are blended into a slurry. The slurry is then atomized from the nozzle at a high rotating speed and dried with hot air to evaporate the water within. The fine powders are thus bonded with each other by the binder to form granulated powders with good flowability. The particle size of the graduated powder is about 40 μm. The granulated powders are filled into the die cavity to produce the green tensile bar using a compacting machine. The tensile bar is debound with the procedure applied from the known arts in the industry. After sintering for 1 hour at 1200° C., the compact is furnace-cooled, and then tempered for 2 hours at 180° C. The sinter-hardened tensile bar has a tensile strength of 1690 MPa, a hardness of HRC47, and a ductility of 3%. These examples show that the sinter-hardening powders disclosed in this invention can be used to produce sintered compacts having improved hardness without undue loss of ductility. The use of MIM method with mixed powders and the use of press-and-sinter method with granulated powders give similar sintered properties. This is expected because the metal powders used in these two methods are the same.

Table 10 shows the properties of some sintered-and-tempered compacts with the compositions (wt %) disclosed in this invention.

TABLE 10 Tensile Sample Hardness, Strength Elongation, No. C Ni Mo Cr Si Mn Others Fe HRC MPa % 47 0.36 8.0 0.8 08 0.3 0.6 the rest 45 1800 3 48 0.34 9.0 0.8 0.8 — the rest 45 1780 4 49 0.40 7.5 0.8 0.5 0.3 — the rest 43 1750 4 50 0.6 8.0 0.8 0.8 0.3 — the rest 48 2010 2 51 0.8 8.0 0.8 0.8 0.3 — — the rest 35 1550 3 52 0.4 8.0 0.8 0.8 — 0.3Cu the rest 46 1950 3 53 0.5 8.0 0.8 0.8 0.2 0.4 — the rest 48 1920 4 54 0.4 8.0 1.0 0.8 0.2 — 0.4V, the rest 46 1960 4 1.2W 55 0.3 12.0 2.0 2.0 0.3 — — the rest 37 1560 6 56 0.4 6 0.5 0.2 0.2 — 1.5Cu the rest 35 1500 2 57 0.5 5.0 1.5 0.8 — — the rest 48 1740 4

Comparative Examples

For comparison, Table 11 shows the compositions and properties of comparative examples which are prepared using the metal injection molding process. MIM 4605 listed in the MPIF standards (sample No. 58) has a strength of 440 MPa after sintering. After quenching and tempering, the strength is 1655 MPa and the elongation is 2% (sample No. 59). The MIM 2700 standard (sample No. 60) yields a strength of 415 MPa and a hardness of HRB69 (too soft to be measured in HRC scale). Sample No. 62 and Sample No 67 have compositions disclosed in U.S. Pat. No. 7,163,569. The hardness and tensile strength of these two parts are lower than those attained using the sinter-hardening powder disclosed in the present invention. Sample No. 70 has a composition according to the Japanese Industrial Standard SCM415. The hardness of the sintered compact is lower than HRC20. Sample No. 71 has the same composition as the AISI 4140 and the hardness obtained is HRC21. Sample No. 72 follows the composition of SAE 8740 and the hardness of the sintered compact is HRC27.

Table 11 shows the compositions (wt %) and properties of comparative examples.

TABLE 11 Tensile Comparative Ni Mo Mn Si Cr Others C Strength Sample No. wt % wt % Wt % wt % Wt % wt % wt % Hardness MPa Elongation % 58 MIM 4605 2.0 0.35 — <1.0 — — 0.5 HRB62 440 15 59 MIM4605⁽¹⁾ 2.0 0.35 — <1.0 — — 0.5 HRC48 1655 2 60 MIM2700 7.5 <0.5 — <0.1 — — <0.1 HRB69 415 26 61 1.75 1.0 — 0.3 — — 0.35 HRC10 650 9 62⁽²⁾ 1.75 2.0 — 0.3 — 0.5Nb 0.34 HRC20 820 8 63 1.75 5.0 0.3 — 0.32 HRC25 1050 6 64 2.0 0.8 0.6 0.3 0.8 — 0.45 HRC29 1120 4 65 2.0 0.8 0.8 — — 0.45 HRC21 990 7 66 2.0 0.8 — 0.3 — — 0.45 HRC11 720 8 67⁽²⁾ 4.0 — — 0.3 — 0.8Nb 0.45 HRB86 583 5 68 4.0 0.8 0.3 — — 0.6 HRC35 1130 3 69 4.0 0.8 — 0.3 0.8 — 0.53 HRC40 1270 6 70 SCM415 0.08 0.22 0.7 0.3 1.05 — 0.18 <HRC20 >410 >15% 71 AISI4140 — 0.21 0.8 0.23 0.96 — 0.56 HRC21 72 SAE8740 0.64 0.31 0.09 0.45 0.49 — 0.26 HRC27 880 12 73 6.96 — 0.04 0.21 0.01 — 0.26 HRC27 880 12 Notes: ⁽¹⁾after quenching and tempering. ⁽²⁾disclosed in U.S. Pat. No. 7,163,569

In conclusion of the above description, the sinter-hardening composition of the present invention can attain easily a hardness greater than HRC30 in sintered compacts that are furnace-cooled. Compared to the best injection molding alloy, MIM-4605 (after quenching and tempering), and the best sinter-hardening alloy (using a cooling rate of a minimum of 30° C./min), FLC2-4808, for the press-and-sinter work piece, listed by the MPIF, the sinter-hardening composition of the present invention can attain similar or even better mechanical properties without the quench-hardening treatment. In addition, the problems derived from quench-hardening in the prior art, including deformation, inconsistency of the dimensions, and cracking after quenching, etc, can be avoided in the present invention, and the costs from the quench-hardening process can be eliminated. Although sinter-hardening alloys have been available for the traditional press-and-sinter process, the cooling rate required for the sintered body is higher than 30° C./min. In contrast, the cooling rate required for the composition of this invention can be as low as 3° C./min. The sintered body of the present invention provides excellent mechanical properties, and it also provides advantages in the areas of dimensional control and lower costs.

The present invention has solved the above-mentioned problems as a result of the carefully selected combination of the base powder and the optimum amounts and types of the alloying elements. The powder mixture or granulated powder uses elemental iron powder, such as atomized, reduced, or carbonyl iron powder, as the base powder. The mean particle size is 20 μm or less. The alloying elements include 0.1-0.8 wt % of C, 5.0-12.0 wt % Ni, 0.1-2.0 wt % Cr, 0.1-2.0 wt % of Mo. The above composition can further contain at least one other minor strengthening element in the amount of 5.0 wt % or less. The strengthening elements can be selected from the group consisting of Cu, Mn, Si, Ti, Al, and P. The carbon can be provided by adding graphite or carbon black, or using carbon-containing carbonyl iron powders. The mixed powder is intended for the production of MIM compacts. The granulated powder is intended for the production of press-and-sinter compacts. Furthermore, the present invention provides a sintered compact by using the powder mixture or the granulated powder. The compact can be sinter-hardened at a normal furnace cooling rate of 3-30° C./minute in the sintering furnace without the need for using fast cooling, which is required for other sinter-hardening powders. The sintered compact does not require any quenching treatment. Only low temperature tempering is needed to obtain optimum mechanical properties. The sintered body with the above-mentioned sinter-hardening powder has unexpected hardness, tensile strength, and good ductility. Since no quenching process is required, the production cost is lower. A higher production yield is also attained due to the elimination of defects, such as cracks and distortions, which occur during quenching.

In actual applications, the sinter-hardening raw powder can be formed as a granulated powder for sintering. Although the mean particle size of the raw powder is 20 μm or less, the mean particle size of the granulated powder can be between 20-150 μm, for example. Further, the raw powder or the granulated powder can be used to manufacture into a sintered compact.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents. 

1. A sinter-hardening raw powder comprising iron (Fe) as a primary composition and further comprising 0.1 to 0.8 wt % of Carbon (C), 5.0 to 12.0 wt % Nickel (Ni), 0.1 to 2.0 wt % chromium (Cr), and 0.1 to 2.0 wt % molybdenum (Mo), wherein a mean particle size of the raw powder for sintering is 20 μm or less.
 2. The sinter-hardening raw powder according to claim 1, further containing at least one element from among 2 wt % or less Cu, 1 wt % or less Ti, 1 wt % or less Al, 1 wt % or less Mn, 1 wt % or less Si, and 1 wt % or less of P.
 3. The sinter-hardening raw powder according to claim 1, containing 0.3-0.7 wt % of Carbon (C), 6.0-10.0 wt % Nickel (Ni), 0.3-1.5 wt % chromium (Cr), 0.2-1.0 wt % of molybdenum (Mo).
 4. The sinter-hardening raw powder according to claim 2, containing 0.1-1.0 wt % of Copper (Cu), 0.2-0.8 wt % of Manganese (Mn), and 0.1-0.5 wt % of Silicon (Si).
 5. The sinter-hardening raw powder according to claim 1, wherein a source of the carbon is from carbon-containing carbonyl iron powder.
 6. The sinter-hardening raw powder according to claim 1, wherein a source of the carbon is from carbon black powder.
 7. The sinter-hardening raw powder according to claim 1, wherein a source of the carbon is from graphite powder.
 8. The sinter-hardening raw powder according to claim 1, wherein the raw powder is elemental powder or ferroalloy powder or a mixture of the two.
 9. The sinter-hardening raw powder according to claim 1, wherein the iron is from carbonyl iron powder.
 10. The sinter-hardening raw powder according to claim 1, wherein the iron is from atomized iron powder.
 11. The sinter-hardening raw powder according to claim 1, wherein the iron is from reduced iron powder.
 12. A sintered compact, comprising compositions of the sinter-hardening raw powder according to claim
 1. 